This study is concerned with 1) the role of thyroxine on the biosynthesis of liver enzymes and organelles involved in urea biosynthesis during differentiation and metamorphosis of the ammonotelic tadpole to the ureotelic frog; 2) the biochemical and other changes of liver nuclei and mitochondria following thyroxine exposure; 3) comparative properties, amino acid sequences and nature of catalytic sites in ornithine transcarbamylases isolated from bovine liver, Strep. faecalis and Strep. durans; 4) the nature and transport of the precursor(s) formed extra-mitochondrially and converted to active carbamyl phosphate synthetase in the mitochondrial matrix volume in liver of tadpoles exposed to thyroxine.